1. Field of the Invention
The invention relates to a method for reinforcing a die, which is based on a surface modification method using an electron beam, and a sulphonitriding treatment. Particularly, the invention relates to a method for reinforcing a die made of a high speed tool steel for hot dies including a matrix high speed tool steel, an alloy tool steel for hot dies, or a stainless steel, and to a reinforced die obtained with the method.
2. Description of the Related Art
A nitriding method or a carbide coating method is conventionally well-known as a method for reinforcing a die. In addition, a surface modification method is known in which a surface of an irradiated object is irradiated with an electron beam of low energy density that has an electron column having a relatively large cross-sectional area. Hereinafter, such method is simply called large-area electron beam irradiation. With the surface modification method utilizing large-area electron beam irradiation, a uniform fine crystal structure layer with no segregation, which cannot be obtained by polishing, is formed on a surface of a metal material.
A typical structure of an electron beam surface modification apparatus that performs large-area electron beam irradiation is disclosed in, e.g., Patent Document 1. Specifically, in the surface modification method utilizing large-area electron beam irradiation, after a chamber is pressure-reduced to a nearly vacuum state, an argon gas is dispersed at a low concentration, and while the argon gas is converted into plasma, accelerated electrons pass through the plasma region at high speed to collide with the irradiated object. The surface of the irradiated object is melted due to the impact and the sharp temperature rise caused by the collision between the electrons and the irradiated object, and the contained substance is eluted on the surface due to heat and surface tension and then re-solidify. Thereby, the surface gets smoother. Moreover, the contained substance is one or more chemical substances added to a die material to provide specific properties.
In the surface modification method utilizing large-area electron beam irradiation that intermittently irradiates a low-energy-density electron beam for about 2 μsec, only a portion of the irradiated object from the surface to a depth of 2 to 5 μm is increased in temperature, and the portion at a larger depth is not melted. Therefore, even if a high-energy electron beam is repeatedly irradiated several times, the shape of the irradiated object is unlikely to collapse, and the entire irradiated surface of the irradiated object is uniformly modified, so that a high quality smooth surface having excellent durability is obtained.
Patent Document 2 discloses a method for reinforcing a die in which a die material as the irradiated object is repeatedly irradiated with a large-area electron beam in a short time to obtain a uniform re-solidified layer and is then subjected to nitriding or carbide coating. With the invention of Patent Document 2, the strength of the die can be improved.
A surface of a die formed by surface modification utilizing large-area electron beam irradiation followed by nitriding has durability and abrasion resistance from the modification utilizing large-area electron beam irradiation, and has durability from the hardening due to the nitride compounds generated by the nitriding treatment. However, how strongly a nitride layer being a layer of nitrogen compounds bonds with a bonding surface is uncertain.
Naturally, in the nitriding treatment, since the nitride layer and a diffusion layer of nitrogen are formed in a thickness of at least a few tens of micrometers, small cracks, so-called “thermal cracks,” are likely to be caused by compressive residual stress that acts on the nitride layer, and there is a fear that the nitride layer having thermal cracks may fall off from the surface of the die. For this reason, although the resistance is increased and durability is improved during the time the surface modification layer is maintained, if the die is continuously used for long time, the surface modification layer will be rapidly lost at a certain time point so that partial peel-off of the surface layer or damage to a curved portion or a corner portion is likely to occur.